1. Field of the Invention
The present invention relates to a method of producing micromechanical components having a cavity which is etched out in a sacrificial layer.
During the production of micromechanical components which can be produced, for example, in the course of a CMOS process, it is often necessary, for example in the case of pressure sensors, to etch out a cavity in a sacrificial layer under a polysilicon layer provided as a membrane or the like. The etching holes in the polysilicon layer are normally arranged in a grid with a typical hole interval of 5 xcexcm. Larger intervals lead to unacceptably long etching times and an unacceptably strong etching attack on the layers which are intended to form the structure to be produced and which must be retained. If the etching holes need to be closed, for example in the case of pressure sensors, the sealing materials cause inhomogeneities in the membrane and make the long-term stability of the sensor worse. When such membranes are used as acoustic isolators for bulk acoustic wave resonators, thin, large-area membranes are required on which no additional closure layers may be applied. Underetching radii of more than 10 xcexcm have until now not been practicable in a silicon/silicon oxide material system.
German published patent application DE 43 36 774 A1 discloses a method for producing micromechanical structures, in which etching openings for underetching the moving micromechanical structure are provided only in its edge region, and in which the underetching process is accelerated by means of channels 15 provided in the sacrificial layer. Before two substrates are connected to produce an SOI substrate, these channels are produced in the insulation layer on the upper face of one of the substrates being used.
It is accordingly an object of the invention to provide a production method for micromechanical components having a cavity under a membrane layer, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and by means of which the membrane layer can be produced to be sufficiently homogeneous using the conventional etching methods.
With the foregoing and other objects in view there is provided, in accordance with the invention, a production method for a micromechanical component having a membrane layer, at least part of which is arranged above a cavity, the method which comprises:
in a first step, applying a sacrificial layer to an upper face of a substrate;
in a second step, applying an auxiliary layer composed of a material with respect to which the sacrificial layer can be selectively etched to the sacrificial layer;
in a third step, forming openings in the auxiliary layer;
in a fourth step, etching channels in the sacrificial layer via the openings in the auxiliary layer;
in a fifth step, applying a planarization layer and closing the openings;
in a sixth step, applying a membrane layer;
in a seventh step, forming etching openings in an edge region of the membrane layer; and in an eighth step, removing the,sacrificial layer, the auxiliary layer, and the planarization layer in a region of a cavity to be produced, via the etching openings and the etching channels produced in the fourth step.
In addition, the method 2, may comprise producing the sacrificial layer and the planarization layer from silicon oxide, and producing the auxiliary layer from silicon, and thermally oxidizing the silicon of the auxiliary layer between the fourth step and the fifth step.
The method according to the invention differs from the conventional methods in that etching openings are provided over an etched-out cavity in an edge region in that part of a structure layer which is intended to be used as a functional element, for example as a membrane, and is referred to as standard in the following text as a membrane layer, in which case this edge region, measured from the side wall or boundary of this cavity, is at most one tenth of the diameter of the functional element, for example of the membrane. This ensures that four fifths of the membrane is free of inhomogeneities in any direction running through the center of the membrane.
On the basis of this structure, the component can be produced using the conventional etching methods for producing micromechanical components having a cavity in a sacrificial layer. To this end, a cohesive network composed of cavities in the form of channels is created in the sacrificial layer on which the membrane layer is applied. This is done even before the membrane layer is deposited. After this, etching holes are produced in the said edge region of the membrane layer. The sacrificial layer is removed to the intended extent, with the etching medium propagating through the cavities which are in the form of channels sufficiently quickly that the sacrificial layer is removed over the entire region of the cavity which is to be produced, but the etching attack does not go beyond the intended boundary of the cavity, or even damage the membrane layer. The channel-like cavities can be produced by etching out a sacrificial layer through openings in an auxiliary layer applied to it. Before the membrane layer is deposited, the auxiliary layer is preferably made smooth by means of a planarization layer, by which means the etching openings are also closed.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of producing micromechanical components, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.